Electroluminescent lamp



Aug. 19, 1958 E. L. MAGER ELECTROLUMINESCENT LAMP Filed Aug. 22, 1950INVENTOR. Ema ,L OWELL MAGER BY? I A-r'roRvEy rates UniteELECTROLUMINESCENT LAMP Application August 22, 1950, Serial No. 180,7 85

2 Claims. (Cl. 313-108) This invention relates to electric lamps of thetype in which light is produced by the application of a varying electricfield to a dielectric material containing a phosphor. The phosphor isembedded in the dielectric material and placed in the field between twoconductors.

I have found that the use of an embedding material of dielectricconstant high with respect to that of the phosphor decreases the voltagenecessary for exciting the device to a desired brightness, provided thatthe resistivity of the dielectric material is also high with respect tothat of the phosphor. A high dielectric constant is not sufiicient ofitself, as shown by the ineflectiveness of glycerin, which has adielectric constant of about 56 but a resistivity of only aboutohm-centimeters, a value high enough for many purposes but not highenough for use in the devices described herein.

Moreover, I have found that while high dielectric constant and highresistivity are necessary in the embedding medium for good brightness,the further feature of low acidity is essential to long life of thedevices. For example, the life of one of my devices with plasticizednitro cellulose as the embedding dielectric was increased from about 50hours to more than a thousand hours by the addition to the nitrocellulose of a small percentage of sodium bicarbonate as a neutralizingagent. Dielectrics, having acid numbers less than 3, such ashydrogenated castor oil, a wax, are sufficiently low in acidity to givegood life without the addition of neutralizing material. The exactreason for the effect of the acidity on life is not known. The effectmay be due to a reaction of the acid with the phosphor or with theconductive plates, or to some entirely diiierent phenomena.

In a specific embodiment of the invention described below, a plate oftransparent conductive glass or plastic is coated with a layer ofphosphor and dielectric, over which a metal layer is placed. The metallayer may be coated onto the phosphor-impregnated layer by variousmethods, but I prefer to apply the metal by vacuum-deposition, whichgives superior results. Depositing the conductive metal film on thephosphor suspension in vacuum has the elfect of insuring intimatecontact of the film and the suspension and of excluding occluded gasthat might otherwise be trapped between the film and the phosphorsuspension. The light output during the life of the resultant lamp canoften be improved by heating the phosphor coating in vacuum for a shortperiod before deposition.

The metal coating can be replaced by an additional plate of conductiveglass or conductive plastic if desired, and other geometricalconfigurations can be used.

Where a metal backing layer or plate is used, aluminum, chromium, oreven stainless steel can be used. These are good reflectors and havegiven satisfactory life.

If the phosphor is used without any embedding material, the glow appearsto be at best rather dim, and to occur only at scattered points ofmetallic contact with the phosphor. The addition of the embeddingmaterial can be seen microscopically to spread the glow out over thesilt.

crystal and increase the brightness. But too much embedding materialwill reduce the brightness below its maximum, and I have found that withhydrogenated castor oil for example, the maximum brightness ordinarilyoccurs With the amount of the plasticized wax present by weight equal toabout twice the weight of phosphor used.

The light output from my devices increases with the voltage applieduntil the breakdown point is reached, so the embedding medium shouldhave a high breakdown voltage, often called its dielectric strength.

Other featuers and advantages of the invention will be apparent from thefollowing detailed description of specific embodiments thereof.

In the accompanying drawings, three devices embodying aspects of theinvention are shown, Fig. 1 being a perspective view partly in sectionof one such device; Fig. 2 being a perspective view of a second device;Fig. 3 being an enlarged cross-sectional view of the device in Fig. 2;Fig. 4 being a plan cross-sectional view of a third device along theline 4-4 in Fig. 5; and Fig. 5 being an enlarged cross-sectional view ofthe device in Fig. 4, along the line 55.

The device shown in Fig. 1 has a glass plate 1, having a transparentconductive surface 2, over which is a thin layer 3 ofphosphor-impregnated dielectric material, with a metal backing layer 4over that and in intimate contact therewith. This completes anillumination source, suitable for use as a luminous plaque for Walls andceilings, for example. One terminal of a proper source of varying oralternating voltage can be connected to a metal backing layer 4, theother to a metal tab 5 which is connected to the conducting surface 2.

In a modification of this device the layer 4 can also be of a conductiveglass, instead of being of metal, thus providing a plaque which emitslight from both sides, and when not energized is translucent. Such adevice can be used in various ways, for example in table lamps and otherlighting fixtures or even as a window pane which transmits sunlight byday and emits its own light at night.

A conducting surface 2 of good transparency or translucency is diflicultto obtain, because good electric conductors are generally goodreflectors of light, rather than transmitters of it. However, althoughother coatings may be used, I find that a particularly effectiveconductive surface can be provided by heating the glass and exposing itwhile hot to vapors of the chlorides of silicon, tin, or titanium, andafterward placing the treated glass in a slightly reducing atmosphere.Where the application in the vapor state is not convenient, good resultsmay be obtained by mixing stannic chloride with absolute alcohol andglacial acetic acid and dipping the glass plate into the mixture.

However applied, the resultant conductive surface appears to containstannic (or silicic or titanic) oxide, probably to some extent at leastreduced to a form lower than the dioxide, although the exact compositionis difficult to determine.

The conductive surface 2 so applied will have a resistance of about ohmsper square, that is a resistance of 100 ohms taken between the entireopposite sides of any square on the surface 2.

The phosphor-impregnated layer 3 placed over the transparent conductivelayer 2 is a phosphor of copper activated zinc sulphide as describedbelow, in the form of fine particles embedded in plasticizednitro-cellulose, and the backing layer 4 is of metal, preferably a goodreflecting metal such as aluminum or chromium, which will not reactappreciably with the phosphor or embedding material used. The metallayer or conductive surface 4 is preferably of low resistance and can beapplied in any convenient manner, taking care not to damage thecellulose-phosphor layer. However, best results have been obtained 'byvacuum-deposition of the metal. The

glass plate 1, with its conductive surface 2, is coated with theembedded phosphor layer 3, placed in a bell jar and the latterevacuated. The coating 3 is then heated for a moment, for example bypassing a current through the conductive'surface 2. The heating ispreferably of-the order of that used for drying, and should not, ofcourse, be sufl'icient to char the embedding material in phosphor layer3. The heating is not essential to producing a plaque of good initialbrightness, but aids in maintaining sion for example including. thefollowing materials:

Phosphor i i g Sodium bicarbonate g 2 Nitrocellulose (quarter-secondviscosity) g 8 Castor oil g l Orthonitrodiphenyl 3 Di-octyl sebacateChlorinated diphenyl Di-octyl phthalate Benzophenone Methyl acetylricinoleate Cellosolve ricinoleate Glyceryl tri-aceto ricinoleate Butylacetate c The substances appearing between nitro-cellulose and butylacetate in the above list are plasticizers for the nitro-cellulose. Thisparticular combination of plasticizers gives a more vitreous mixture,with less tendency toward crystallization. Other plasticizers can beused, however, and a single one will often suffice. The butyl acetate isa solvent for the nitro-cellulose and plasticizers. Other solvents ormixtures thereof can be used.

The plasticizer will generally be a large proportion of the embeddingmaterial, because the electroluminescence of the phosphor appears to bebest when the embedding material is soft.

The plasticizer should have high resistivity, high dielectric constantand high dielectric strength, in order not to greatly diminish thevalues of these factors in the main cellulosic component.Chlorinated-di-phenyl, for example, has a dielectric constant of about6. Nitrocellulose itself has a constant of 7.5.

The proportions of dielectric material to phosphor in the aboveexample(after drying) are seen to be about 2.5 to 1 by weight. Proportionsbetween 2 and 3 are generally satisfactory for best results, althoughthe ratio can be varied considerably.

The sodium bicarbonate prolongs the life of the lamp, presumably byreducing or neutralizing the acidity of the embedding material. 0

In making the above suspension, the nitro-cellulose is dissolved inabout cc. of the butyl acetate and this lacquer is then added to therest of the mixture. The suspension is sprayed onto the conductingsurface 2 of glass plate 1 with a spray gun ofa type usual in the art.

The coating is air-dried, or oven-dried, and on its exposed surface athin layer of m etal, for example, aluminum is deposited by the usualevaporation methods .or by some other method, as previously explained.

Various other plastics can be used instead of nitrocellulose. Glass andvarious enamels may be used, particula'rly "glass oflow enough meltingpoint to insure that the phosphor cr'ystalsremain unmelted The voltageat which the layer 3 has a given brightness, can be reduced by usinghydrogenated castor oil, or-

2,848,637 7 g W p i f castor wax, as the embedding medium. This wax hasan acid number of 2 and a dielectric constant of about 12. A suitableembedding medium and phosphor combination with this type of wax is asfollows:

Grams Phosphor 20 Castor wax 24 Beef fat 12 Castor oil 4 into suspensionfor a moment, and then freezes them while suspended. A longer heatingmay, allow ticles to settle out of suspension.

The beef fat and castor oil are plasticizers, and although otherplasticizers can be'used, the combination the parof these two isparticularly satisfactory, presumably because of their dielectricproperties.

The thickness of the various layers can be altered to The volt-' suitvarious voltage conditions and the like. age necessarily will depend onthe phosphor used, the thickness of the phosphor layer 3, and thebrightness desired, but voltages between 25 volts and 2500 volts andeven higher have been used. A lamp operable from a llO-volt alternatingcurrent power line can be made with the conducting surface 2 of athickness of about a wave length of light, producing an iridescenteflect when viewed at an angle, the phosphor layer 3 of about 2one-thousandths of an inch, and the metal layer 4 of a fraction of athousandth of an inch. The plate 1 can have any convenient thickness andshould be transparent ortranslucent.

The phosphor used can be made, for example, by inti-l mately mixing asfine powders about 75 parts of zinc sulphide and 25 parts zinc oxide,with about 1 part zinc chlo-' ride, 1 part lead sulphate and about 0.075part copper sulphate.

The mixture is then heated at between 900 C. and 12509 C. in an inertatmosphere, for example in a gas tightfurnace through which nitrogen isflowed, and removed from the furnace after the evolution of fumes hasdecreased but before the phosphor darkens too much and becomes gritty.The phosphor will generally be a light greenish-gray in color.

It should then be treated with an aqueous solution of 5% acetic acid atbetween 60 C. and C., then washed with Water.

until the final washing with pure Water.

The preparation of this phosphor, and of other electroluminescentphosphors useful in my lamp, is described in detail in an applicationfiled by Elmer C. Payne concurrently herewith for an ElectroluminescentLamp.- Other phosphors will be effective in my device however, and mydevice is not restricted to the use of those described.

The voltage at which the layer 3 has a given brightness may be reducedby using castor wax (hydrogenated castor oil) as the embedding medium.Castor Wax has an acid number of 2 and a dielectric constant of about12.

The voltage necessarily will depend on the phosphor used, the thicknessof the phosphor layer 3, and the bright:

, ness desired, but voltages between 25 volts and 2500 volts and evenhigher have been used. Lamps, according to my invention, operatingdirectly from the usualllO volt An aqueous solution of half-satu ratedammonium acetate can be used instead of the acetic acid, if desired, andgradually reduced in concentration line, with no transformers orauxiliary equipment necessary, have been made.

The device can also be user-l in the forms shown in Figs. 2 to 5, inwhich paired long spaced narrow conductors 6 and 7, and 8 and 9, areplaced side by side, the conductors and the space between them beingoccupied by a coating or layer 10, 11 consisting of anelectroluminescent phosphor embedded in a dielectric material. Theconductors and the layer are carried by insulating supports 12 and 13.In Fig. 2 the conductors 6, 7 are Wires, having an enamel insulatinglayer 14, wound side by side and close together but spaced apart adistance of a few thousandths of an inch or less. In Fig. 4, theconductors 8 and 9 have intermeshed lengths which can be metal paintedor suitably adhered to an insulating support such as the glass plate 13.The opposite sets 8, 9 of conductors, While intermeshed, are not incontact with each other, being spaced apart a few thousandths of an inchor less.

A lamp is defined for the purposes of this specification as a devicewhich produces light of practical illuminating intensities. Intensitiesbelow a foot-lambert are practical for some application, although thelamps herein described have given several foot-lamberts on 60 cycles persecond alternating voltage supply, and 15 to 20 foot-lamberts on asupply of several thousand cycles per second.

Such lamps are therefore useful for general illumination purposesincluding use as luminous panels for ceilings, as lighting sources fortable lamps, as luminous signs and clock faces, as luminous face platesfor household electrical switches, for street lighting and for manyother 30 applications.

This is in part a continuation of my copending applica tions Serial Nos.120,398 and 141,050, filed October 8, 1949, and January 28, 1950,respectively, now Patent No. 2,624,857 and Patent No. 2,566,349,respectively.

What I claim is:

1. An electroluminescent lamp comprising two spaced conductors in closeproximity to each other and a coating therebetween of phosphor embeddedin a Wax of hydrogenated castor oil.

2. An electroluminescent lamp comprising two spaced conductors in closeproximity to each other and a coating therebetween of phosphor embeddedin an insulating material the dielectric constant and resistivity of theembedding material being high compared to that of the phosphor.

References Cited in the file of this patent UNITED STATES PATENTS1,651,398 Lorenz Dec. 6, 1927 2,459,633 Farris Ian. 18, 1949 2,559,279Charles July 3, 1951 OTHER REFERENCES G. Destriau: The New Phenomenon ofElectrophotoluminescence and its Possibilities for the Investigation ofCrystal LatticePhilosophical Magazine, October 1947, vol. 38, pps. 700,701, 702, 711, 712, 713, and 723. especially p. 713.

